1. Field of the Invention
The present invention relates to a system and method for operating a power supply. More specifically, the present invention relates to a system and method for utilizing battery power sources in an electronic power supply.
2. Background
Conventional electronic power supplies that operate using battery power sources suffer from a number of disadvantages.
For example, a conventional method for providing a DC voltage to a DCxe2x80x94DC converter utilizes two or more batteries in a series configuration. Often, when batteries are configured serially, one battery will become depleted well in advance of the others. This is due, in part, to varying internal series resistances as well as other characteristics that may cause each battery to discharge at a different rate. Where one battery dies in advance of the others in a series configuration, no power can be delivered to the device being powered.
Recharging batteries that are in series can also be problematic. Because the batteries in a stack typically discharge at different rates, the voltage in each battery before recharging will be different. If one of the batteries in series has been severely discharged to the point where gas has started to build on the anode or cathode, that battery can actually reverse polarity. When an attempt is made to recharge the batteries, the battery with a reverse polarity will, in effect, be charged in reverse. This will result in the death of that battery, which means that no power can be delivered to the device being powered as discussed above. Additionally, it has been observed that charging a battery with a reverse polarity can actually cause the battery to be damaged or heat up to the point where it will explode.
Furthermore, when batteries are arranged in series, it is difficult to monitor how much charge is in each of them. Because batteries are manufactured with slight differences, each individual battery will charge and discharge at a different rate. It would be useful to know for discharging and recharging purposes how much charge remains in each battery in a battery pack. In a conventional series arrangement, the overall voltage coming out of a battery pack may be detected, but it is impossible to determine the state of one of the cells in the middle of the stack.
Conventional schemes for operating two or more battery cells in parallel are also disadvantageous in that they require the addition of external parts to add multiple cells to a battery pack, which can be complex and costly. Moreover, conventional techniques for operating cells in parallel do not avoid the problem of over-discharging, or xe2x80x9cdeepxe2x80x9d discharging of a battery that can result in polarity reversal.
A further disadvantage of conventional battery-operated power supplies is that they utilize separate power control chips that must be coupled to the application to be powered using external logic and parts. This makes the design more expensive. Furthermore, because these separate power control chips are not xe2x80x9con chipxe2x80x9d with the application being powered, they do not have access to a priori information about impending load changes. Thus, conventional battery-operated power supplies require large load capacitors to act as charge buffers to prevent sudden load changes from pulling a supply voltage out of specification (in the case where a large load is suddenly presented) or from causing the supply voltage to spike too high (in the case where the load current is suddenly turned off).
Conventional battery-operated power supplies also do not monitor the state of batteries by determining how much charge is in them, but instead simply look at the voltage on the battery. Although the voltage on the battery provides an indication of the state of the battery, that information is not as useful as tracking how much charge remains in the battery.
Conventional battery-operated power supplies are also disadvantageous in that they typically utilize a Schottky diode to discharge current into a load. Because a Schottky diode has a typical turn-on voltage of about 0.4 to 0.6 volts, the use of the diode will result in an energy loss equal to the turn-on voltage times the load current. This is lost energy that could have otherwise been used by the load. Where only one or two small batteries are being used, this loss can be quite significant. The use of Schottky diodes is also problematic because it is impossible to control their turn-on and turn-off characteristics beyond their manufactured values.
There is a great need for innovative techniques that would enable portable electronic equipment to operate more efficiently from battery power sources. For example, in the Bluetooth(trademark) area of wireless products, it is anticipated that there will be great demand for Bluetooth(trademark)-compatible headsets. High efficiency and small size are essential to the success of any Bluetooth(trademark) headset. A battery-operated power supply is used to power the headset at the correct voltage from rechargeable batteries.
The present invention provides a system and method for drawing charge from two or more batteries under the control of a power controller, wherein the power controller may control and monitor the amount and rate of discharge. The present invention may also be used to recharge two or more batteries under the control of a power controller, wherein the power controller may control and monitor the amount and rate of charging. In an embodiment, the present invention may be used to provide power to a DC to DC converter and to recharge batteries used for providing power to a DC to DC converter.
The present invention is advantageous in that it provides a separate connection to each battery for charging and recharging purposes. As a result, the depletion of any single cell in a battery pack will not result in a complete loss of power to the application being powered, as charge may be drawn from any of the other cells that have not yet been depleted. In fact, the power controller of the present invention may be used to increase the power drawn from the surviving cells to compensate for the loss of the depleted battery.
Because embodiments of the present invention provide a separate connection to each battery, the state of each battery may be monitored on an individual basis. As a result, each cell can be charged and discharged in an optimal way. For example, polarity reversal may be avoided by tracking the amount of charge that has been drawn off of each battery in the battery pack. Additionally, discharging and recharging may be evenly applied to each cell. Optimal charging and discharging of each battery will permit the battery to last through more charge/discharge cycles, hold more power before needing charging, and be far more reliable than batteries in conventional power supplies.
Also, because each battery may be monitored on an individual basis, embodiments of the present invention also permit information to be provided to the user about the state of each battery, such as a warning when a battery""s charge is low or when a battery is depleted.
In an embodiment of the present invention, the state of each battery is monitored by utilizing counters that maintain a continuous running total of how long the battery has been charged or discharged. This information, in turn, may be used to determine the precise amount of charge remaining in each battery. This technique represents an improvement over conventional power supply circuits, in which the status of a battery or batteries is determined simply by looking at the voltage of a battery or batteries.
The present invention is also advantageous in that it permits the addition of multiple cells in a battery pack with a very minimal external parts count, while still keeping the cells separate so that charging can be optimized and so that the depletion of individual cells dying in the battery pack will not cause a loss of power to the entire product. As a result, embodiments of the present invention may be implemented in an easy and inexpensive manner.
Another benefit of the present invention is that it may be implemented very efficiently on-chip along with an application circuit being powered. For example, an embodiment of the present invention is implemented on-chip with an application circuit in CMOS. Since an embodiment of the present invention may be implemented in digital CMOS, digital logic may be easily added to the power controller that will allow fine tuning of the rate of charge/discharge of a given battery in light of battery condition and load demands.
Also, since embodiments of the present invention may be implemented on-chip with the application circuit being powered, the application circuit can provide useful information to the power controller to help control the power supply. For example, the application circuit can provide the power controller with a priori information about an impending load change. This may occur, for instance, where the application circuit is going from a standby state to an active state, or vice-versa. The access to information regarding impending load changes permits the power controller to control the power supply to increase or decrease the power supply accordingly in advance of the change. As a result, embodiments of the present invention do not require the large load capacitors of conventional designs that act as charge buffers which compensate for large and sudden load changes.
Additionally, embodiments of the present invention use a controlled FET to discharge current into a load, and thus avoid the energy loss associated with conventional designs that use a Schottky diode to perform that function. This is due to the fact that a FET may have a drain-to-source voltage that is significantly lower than the turn-on voltage of a Schottky diode. For example, a FET with a drain-to-source voltage of 10 mV may be used as compared to a Schottky diode with a typical turn-on voltage of 0.4 to 0.6 V. Furthermore, the use of a FET in embodiments of the present invention permits precise control of turn-off and turn-on voltages as opposed to Schottky diodes, in which the turn-on and turn-off voltages cannot be modified after manufacture.
Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the system and method particularly pointed out in the written description and claims hereof as well as the appended drawings.